I'll tell you one thing that many of us wish would work but usually doesn't: Saying things like, "Stop that!" or "Don't do that!" from across the room.
When your young child starts using his or her body in a way you are not happy about, you'll probably need to use your body to intervene. If your child is hitting, you'll gently catch his hand and hold it still or push it against something that is okay to hit. If your child is kicking, you'll gently catch her leg and move it in another direction. If your child is throwing, you'll gently aim his arm at a safe target. If your child is flailing around, you'll gently hold her arms and legs wrapped up in yours until she gathers her self control again.
Notice how often the word gently shows up in those sentences? Please use only exactly the amount of force that is necessary to redirect, contain, or protect, and not one ounce more. In our parenting workshops, Robin and I call it "protective action," a term we borrow from Marshall Rosenberg's Nonviolent Communication model.
The idea is that our job as parents is to keep everyone safe. This includes protecting young children from doing damage to themselves, others, or property while their developing brains are still unable to control their impulsivity. We do a child no favor when we let him hit or hurt us or anyone else. Since he's too young to be reliably in control of his body, especially while he's feeling strong emotion, we need to be his external safeguard.
As we are gently containing, protecting, or redirecting, it can be helpful for us to say, "I will keep us safe." This reminds us that our intention is protection, not punishment. And it lets the child know that we are stepping in for the good of all concerned, including the aggressor.
When a child is acting out physically, it's not a teachable moment. Trying to use words to stop or redirect the behavior at that point is sort of like talking to a reptile. Emotions and learning don't mix. So take protective action until your child is calm enough to listen and talk. Then you can discuss alternative behaviors that will work better for him.